This invention pertains to pressure transducers and particularly to a pressure transducer useful as a manifold absolute pressure sensor (commonly called a MAP sensor) in conjunction with an electronic engine control system for an automobile engine.
As part of the effort toward compliance with governmentally mandated exhaust emissions standards and fuel economy performance standards, electronic control systems have been introduced in recent years on automotive internal combustion engines. These control systems are used for such control functions as spark timing or fuel management and comprise sensors for sensing various operating parameters (such as manifold vacuum, engine temperature, engine speed, etc.) and supplying corresponding input signals to electronic controls which in turn process these signals to develop output signals performing the desired control functions. The electronics technology for these applications has advanced to a point where it is now advantageous to use digital microprocessor-based systems on a mass-production basis for engine control functions. However it is generally fair to say that sensor development has not kept pace with the rapid acceleration in electronics technology. The problem lies not in a lack of any sensors at all, but rather in that development of cost-effective sensors has considerably lagged the progress of cost-effectiveness of microprocessor-based electronics.
A pressure transducer is one type of sensor which is useful in an engine control system. An example is a MAP sensor which senses manifold absolute pressure and provides a corresponding output signal. Many known types of absolute pressure transducers are relatively expensive and inherently sensitive to ambient temperature, such temperature sensitivity generally being caused by the sensitivity of the basic pressure sensing element. For this reason, among others, such transducers are not suited for automotive engine control applications. Typically, such absolute pressure transducers contain capacitive capsules, silicon or germanium strain gauges on or with pressure sensing diaphragms, or multiple coil pickups which are relatively expensive to manufacture. Furthermore, these devices tend to have fixed internal transfer characteristics such as capacitance versus pressure, resistance versus pressure, or peak-to-peak voltage versus pressure, and generally require electronic compensation internal of the transducer to calibrate the slope and offset during manufacturing.
The present invention, in its broadest aspect, relates to an improved pressure transducer well suited for mass-production usage and in the preferred embodiment is disclosed as an absolute pressure transducer for use as a MAP sensor. The principles of the present invention provide a means for overcoming the inherent disadvantages of prior transducers, as described above. The transducer of the present invention can be used to advantage when connected in an electronic circuit of the type shown in U.S. Pat. No. 4,060,714 to perform an analog to digital conversion of absolute pressure. A transducer embodying principles of the present invention requires only a single inductance coil which is relatively inexpensive to produce. No internal compensating electronics are required. A metal shield surrounding the coil attenuates sensitivity to external magnetic fields and increases gain. Such reduced sensitivity and increased inductance change allow the output of the transducer to be fed directly to a circuit such as described in the foregoing patent without signal processing internal to the transducer. This decreases the overall system cost by allowing all electronic components to be integrated into the microelectronics itself. It also reduces the number of sources of signal conversion inaccuracies because of the reduction in the number of interconnected components. By selecting the transducer characteristics to optimize accuracy and stability, the sensitivity to structural deflections within the transducer is decreased, permitting lower cost materials to be used in construction. It also makes the calibration simpler. The transducer provides improved adjustment to set both transducer absolute inductance (i.e. offset) and total inductance change (i.e., slope or span) after the transducer has been fully assembled. Internal mechanical temperature compensation is provided to compensate for mechanical changes in the structural and sensing elements within the transducer. External resistance values can be chosen to minimize temperature effects of coil resistance if the associated circuits are susceptible to resistance variations in the inductance coils.
The foregoing features, advantages and benefits of the invention, along with additional ones, will be seen in the ensuing description and claims which should be considered in conjunction with the accompanying drawings. The drawings disclose a preferred embodiment of the present invention according to the best mode presently contemplated in carrying out the invention.